This invention relates to a nitride semiconductor laser device and particularly, to a laser device having an effective refractive index type narrow stripe ridge structure, which is used in an optical information processing field and can realize a continuous-wave operation at a high power without a kink.
As the information-oriented society has developed in recent years, a xcfx86 device which can store a large amount of information has been required. Particularly, the layer light source having a short wavelength is longed for as a light source for a large amount media such as DVD and as a light source for communication. The applicators reported a nitride semiconductor layer device having a lifetime of ten thousand hours or more under room-temperature continuous-wave operation of in the single mode at the wavelength of 403.7 nm.
As mentioned above, the next problem the nitride semiconductor device which can realize continuous-wave oscillation is to raise the power of the device, in order to put the device to practical use and expand the application fields. And a longer lifetime of stable continuos-wave oscillation must be realized.
Particularly, the semiconductor laser device which is used as a light source for the mass storage optical disk such as DVD requires sufficient optical power to enable recording and regenerating. And in such a semiconductor layer device, the stable lateral mode of oscillation is required. Concretely, the oscillation at 5 mW and 30 mW is required as optical power during recording and regenerating. And under such an oscillation, there is needed no kink in the electric currentxe2x80x94optical power characteristics.
However, when the injection current into the laser device is increased and the optical power is increased, there is generally a kink due to the instability of the lateral mode, subsequently to the linear region after the initiation of oscillation, in the currentxe2x80x94optical power characteristics. In order to put the laser device to practical use, the stable basic single lateral mode without a kink must be achieved in the optical power region of from the initiation of oscillation to 30 mW and further, a longer lifetime of oscillation must be achieved.
In view of the above-mentioned problems, the first object of the present invention is to provide a semiconductor laser device in which the stable lateral mode of the basic oscillation mode can be achieved in the region of from relatively low optical power to the high optical power and there is no kink in the current-optical power characteristics in such a region. The object can be accomplished by the effective refractive index type nitride semiconductor laser device comprising an active layer and at least a p-side cladding layer and a p-side contact layer which are laminated in turn on the active layer, wherein the waveguide region of a stripe structure is provided by etching from the p-side contact layer to above the active layer, characterized in that the stripe width providing by etching is within the narrow stripe range of 1 to 3 xcexcm and the etching depth is above the active layer and below the remaining thickness of the p-side cladding layer of 0.1 xcexcm.
That is, according to the present invention, the waveguide having such stripe width and depth enables the basic mode emission having a stable lateral mode and there is no kink in the wide light output power range, resulting in a semiconductor laser device having a longer lifetime under continuous-wave operation.
Preferably, the insulating film other than Si oxide is formed on the both side surfaces of the stripe which has been exposed by said etching or the flat surface of the nitride semiconductor continuing with said side surfaces, and an electrode is formed on the p-side contact layer which is the uppermost layer of the stripe via said insulating film. Thereby, in such a nitride semiconductor laser device, the insulating property between the positive electrode and p-side cladding layer is improved and particularly, the light output power is enhanced. That is, the effect tends to be remarkable as the driving current increases. Therefore, the device having less leak current and a high reliability can be obtained.
Preferably, the stripe width is 1.2 to 2 xcexcm. This nitride semiconductor device has a refractive index waveguide structure in which the lateral mode is stable in the high light output power region, for example, in the region exceeding 5 mW, therefore, a basic (single) mode emission is further improved to enable the emission without kink in the wide light output power range.
Concretely, said insulating film other than Si oxide may include at least one among an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf and Ta, and SiN, BN, SiC and AlN. Said insulating film gives the device a high reliability.
The waveguide region having a stripe structure in this laser device is formed by the following steps, resulting in the nitride semiconductor laser device having a extremely good precision and a good yield. The method comprises a first step of laminating a p-side contact layer containing the second p-type nitride semiconductor on the p-side cladding layer containing the first p-type nitride semiconductor and thereafter, forming a first protective film in the stripe structure on the surface of the p-side contact layer; a second step of etching the nitride semiconductor in the part on which the first protective film is not formed via the protective film so as to form a waveguide region having a stripe structure just under the protective film; a third step of, after the second step, forming a second protective film made of the material other than that of the first protective film and having an insulating property on the side surface of the stripe waveguide and the flat surface of the nitride semiconductor layers which has been exposed by etching; and the fourth step of, after the third step, removing the first protective film. In this case, as mentioned below, the first protective film may be formed in the desired shape using the third protective film.
And in order to apply the laser device to the laser light source as mentioned above, it is needed to improve the properties, particularly optical properties of the laser device, that is, to improve the waveguide of the semiconductor laser, for example, to improve the aspect ratio and the far-field pattern and prevent the leakage of light. Concretely, said laser device having a longer lifetime is an effective refractive index waveguide type and it is needed to realize a high-precision control of the lateral mode. In the ridge waveguide structure, the effective refractive index changes depending on the etching depth, stripe height and the like. Such a structure change effects on the device properties extremely. Then, the second object of the present invention is to improve the beam shape, that is, the aspect ratio in the F.F.P (far-field pattern). If the laser device is applied to the optical disk system or the laser printer, the laser light is corrected and adjusted by each optical system. In this case, if the aspect ratio is large, the correction optical system is a large-scale one, therefore, the design, manufacturing and the loss via the optical system is a large problem. Further, for the nitride semiconductor light emitting device, the measures to control light leakage which has been a problem since a prior time is needed, because the light leakage appears as a ripple in the laser device and causes the problem of noise in the laser device application.
According to the present invention, the above-mentioned second object is accomplished by the effective refractive index type nitride semiconductor laser device comprising an active layer and at least a p-side optical waveguide layer, a p-side cladding layer and a p-side contact layer which are laminated on the active layer, wherein the waveguide region of a stripe structure is provided by etching from the p-side contact layer to above the active layer, characterized in that said p-side optical waveguide layer comprises a projection part of a narrow stripe structure and a p-type nitride semiconductor layer on the projection part and the projection part of the p-side optical waveguide layer has a thickness of not more than 1 xcexcm.
Thus, the p-side optical waveguide layer having a thickness of not more than 1 xcexcm comprises a projection part of a stripe structure and therefore, the horizontal lateral mode which has not yet realized can be controlled, with the result that the laser emission with a good aspect ratio can be obtained. That is, in this laser device, the laser emission of a good beam shape can be obtained under continuous-wave operation in the single mode. And since the projection part of the n-side optical waveguide layer and the p-type nitride semiconductor layer on the projection part is a stripe ridge waveguide formed by etching from the p-type nitride semiconductor layer, the ridge waveguide can be formed highly productively. And the etching depth is the p-side optical waveguide layer in this case, the laser emission of a beam shape can be obtained.
Preferably, the p-side optical waveguide layer in the projection part has a thickness in the range of not less than 1500 angstroms and not more than 4000 angstroms. This is because both the laser emission of a good beam shape and a good output property can be achieved at the same time. Concretely, for the beam shape, F.F.P. in the horizontal direction (x direction) is not less than 10xc2x0 and good broadening can be achieved. The aspect ratio is within the range where the easy correction by means of the external optical system is possible, so as to make it easy to apply the laser device to optical information equipment.
Further, since the p-side optical waveguide layer has a thickness of not less than 500 angstroms and not more than 1000 angstroms in the region except the projection part, a good stripe waveguide region can be formed and the laser device of a good beam shape can be obtained highly productively. In this case, for the beam shape, F.F.P in the horizontal direction is within 12xc2x0 to 20xc2x0 and the aspect ratio is about 2.0 and moreover, about 1.5.
And since the stripe width of the projection part is within the range of not less than 1 xcexcm and not more than 3 xcexcm, the laser device is obtained in which good single lateral mode emission can be observed.
Further, since the height of the projection part in said p-side optical waveguide layer is not less than 100 angstroms, the laser device of a good beam shape can be obtained. Preferably, the height thereof is not less than 500 angstroms. Single mode emission can be observed even at a high output power. Therefore, enough reliability required for the application of the laser device can be secured.
And the p-side optical waveguide may be made of InxGa1xe2x88x92xN(0xe2x89xa6x less than 1), thereby a good waveguide can be formed, resulting in a laser device having good device properties.
Further, viewed in another light, the optical waveguide layer is focused. The third object of the present invention is to provide a laser device of a good beam shape by making the thickness of the p-side optical waveguide layer different from that of the n-side optical waveguide layer.
That is, the third object of the invention is accomplished by the effective refractive index type nitride semiconductor laser device comprising an active layer and at least n- and p-side optical waveguide layers, n- and p-side cladding layers and n- and p-side contact layers which are laminated respectively on both sides of the active layer, wherein the waveguide region of a stripe structure is provided by etching from the p-side contact layer to above the active layer, characterized in that the thickness of said p-side optical waveguide layer is larger than that of the n-side optical waveguide layer.
According to the present invention, good optical confinement can be achieved and ripples can be decreased without increase of threshold current. And in- the manufacturing steps, when the stripe waveguide and the like is formed by etching and the etching depth is down to the p-side optical waveguide layer, the etching precision can be enhanced compared with in the past.
Preferably, the p-side optical waveguide layer comprises a projection part of a stripe structure and a p-type nitride semiconductor layer on the projection part, and the thickness of the p-side optical waveguide layer is not more than 1 xcexcm. Thereby, the stripe waveguide region can be formed and a good control of lateral mode can be achieved. Concretely, the difference of the effective refractive index is formed effectively, and therefore, the beam shape of the light, particularly the spread of the light of the far-field pattern in the direction parallel to the junction plane can be improved compared with in the past. Also, the aspect ratio is good for the application of the device. Further, the enhancement of the optical properties suppresses the threshold current to the same extent or the more than in the prior and enables the long lifetime emission, while conventional various properties relating to the emission is maintained.
Preferably, the projection part of the p-side optical waveguide layer and the p-type nitride semiconductor layer on the projection part is a ridge waveguide of a stripe structure formed by etching from the p-type nitride semiconductor layer. Since the ridge waveguide is formed in the p-side optical waveguide layer, the threshold current can be decreased and a long lifetime and stable single mode emission can be realized. Moreover, the optical confinement in the lateral mode is good and the laser emission having a good aspect ratio can be obtained.
The p-side optical waveguide layer preferably has a thickness of not less than 2500 angstroms. This is because for in guiding waves of the laser emission, the effective refractive index functions more effectively and the optical confinement in the horizontally lateral mode is better, resulting in the laser emission with a stable and good aspect ratio.
The thickness of the region of the p-side optical waveguide layer except the projection part is preferably not more than 500 angstroms and not less than 1000 angstroms. This is because the laser devices can be manufactured in the stable state, the dispersion of devices is small and the yield of manufacture can be increased.
The stripe width of the projection part is preferably not less than 1 xcexcm and not more than 3 xcexcm. Thereby, good control of the lateral mode can be realized, particularly single-mode emission is possible and the occurrence of kinks can be prevented even at high output power.
According to the present invention, preferably, the p-side optical waveguide layer is made of InxGa1xe2x88x92xN (0xe2x89xa6x less than 1) and a good optical waveguide can be formed.
Further, the fourth object of the present invention is to enhance the output power and reliability of the laser device according to the present invention.
Fourth object of the present invention is to provide an effective refractive index type nitride semiconductor laser device comprising an active layer with at least a p-side first cladding layer, a p-side optical guide layer, a p-side second cladding layer and a p-side contact layer stacked thereon, while the layers from the p-side contact layer to that above the active layer are etched thereby to form a stripe-shaped waveguide region, made by applying the structure of the present invention to a semiconductor laser device having a first cladding layer for carrier confinement and a second cladding layer for light confinement formed from the side near the active layer and an optical guide layer provided between the first cladding layer and the second cladding layer, wherein the p-side optical guide layer has a protruding portion having narrow stripe shape and a p-type nitride semiconductor layer being formed on the protruding portion, and the protruding portion of the p-side optical guide layer has a thickness of 1 xcexcm or less.
While the p-side first cladding layer is a carrier confinement layer made of AlyGa1xe2x88x92yN (0 less than y less than 0.5) and the p-side second cladding layer is a light confinement layer made of AlzGa1xe2x88x92zN (0 less than z less than 0.5: y greater than z), the p-side first cladding layer is preferably made of AlyGa1xe2x88x92yN (0 less than y less than 0.35). The p-side first cladding layer also preferably includes at least two layers, with the first layer being made of AlxGa1xe2x88x92xN formed in nitrogen atmosphere and the second layer being made of AlxGa1xe2x88x92xN formed in hydrogen atmosphere.